1. Field of the Invention
The present invention relates to a torque rotor. More particularly, the present invention relates to a torque rotor and a method for manufacturing the torque rotor, which can prevent inflow of plastic between a yoke and a magnet during a conventional plastic injection molding process for forming an assembling structure after assembling between the yoke and the magnet, thereby preventing degradation of a roundness of the magnet or damage of the magnet due to the difference in the temperature expansion coefficients, and preventing idle rotation of the yoke and the magnet relative to each other.
2. Description of the Prior Art
A vehicle generally uses a power steering apparatus serving as a means for reducing steering power of a steering wheel (handle) and guaranteeing the stability of a steering state. In this respect, a conventional Hydraulic Power Steering system (HPS) using hydraulic pressure has been widely used for the HPS of the vehicle. However, in contrary to the conventional scheme of using the hydraulic pressure, an Electronic Power Steering System (EPS) that makes steering performance of a driver easy using rotational force of a motor and is environment-friendly has been widely installed in vehicles recently.
Such an EPS drives a motor by means of an electronic control unit according to a driving condition of a vehicle sensed in a vehicle speed sensor, steering angle sensor, steering torque sensor, or the like, so that it provides light and comfortable steering sense in low-speed driving, a heavy steering sense and satisfactory direction stability in high-speed driving, and restoration force for promptly restoring the steering wheel according to a rotation angle of the steering wheel. Therefore, the EPS enables the prompt steering in an emergency so that it provides the driver with an optimum steering condition.
Meanwhile, in the EPS, the motor is installed in an external side of a steering column that is located between the steering wheel and a gear box so as to downwardly transfer the rotational force of the steering wheel of the driver so that it rotates a steering shaft within the steering column, thereby complementing steering power of the driver according to the steering of the steering wheel.
FIG. 1 is a view schematically illustrating a conventional electronic power steering apparatus. As shown in FIG. 1, the electronic power steering apparatus 100 includes a steering system 130 extending from a steering wheel 102 to wheels 126 in both sides and an auxiliary power mechanism 140 for supplying steering auxiliary power to the steering system 130.
The steering system 130 includes a steering shaft 106, of which one side is connected to the steering wheel 102 to rotate together with the steering wheel 102 and the other side is connected to a pinion shaft 108 through a pair of universal joints 104. Further, the pinion shaft 108 is connected to a rack bar 112 through a rack-pinion mechanism part 110 and both ends of the rack bar 112 are connected to the wheels 126 of a vehicle through a tie road 112 and a knuckle arm 124.
The auxiliary power mechanism 140 includes a torque sensor 142 for sensing torque applied to the steering wheel 102 by the driver and outputting an electric signal proportional to the sensed torque, an electric control unit 144 for generating a control signal based on the electric signal transferred from the torque sensor 142, a motor 146 for generating auxiliary power based on the control signal transferred from the electric control unit 144, and a reducer 150 including a worm gear 152 and a worm wheel gear 156 for transferring the auxiliary power generated in the motor 146 to the steering shaft 106.
FIG. 2 is an exploded perspective view illustrating the conventional torque sensor.
As illustrated in FIG. 2, the conventional torque sensor 142 includes a torque rotor 220, a position rotor 230, a magnetic charge detection unit 240, the electric control unit, and a housing 255.
The torque sensor 142 measures the torque of an input shaft 180 and an output shaft 140 of the steering shaft and sends the measured torque to the electric control unit. Then, the electric control unit operates a driving motor and supplies the steering auxiliary power. Recently, a contactless torque sensor that measures a variation quantity of the magnetic field to measure the torque variation has been widely used.
The torque rotor 220 includes a rotor body 224 having a magnet 225 provided along an outer peripheral surface of the rotor body 224, an input shaft insertion hole 221 formed in a center of the rotor body 224, a plurality of compression parts 222 protruding along a boundary of the input shaft insertion hole 221, and a compression ring 223 fitted in the compression part 222.
The rotor body 224 is generally manufactured by injection-molding a plastic into a yoke (not shown) having a ring shape, and includes the magnet 225 on the outer peripheral surface thereof, in which the magnet 225 has N poles and S poles that alternately repeat along a boundary of the peripheral surface of the rotor body 224.
Here, the N poles and the S poles are alternately repeated 6 times, 8 times, or the like.
The position rotor 230 includes a torque rotor insertion part 239 in which the torque rotor 220 is seated and an output shaft fitting part 237 to which one end of the output shaft 140 is fitted in a center thereof. An upper magnetic body 236 connected to an upper stator 231 and a lower magnetic body 235 connected to a lower stator 232 are alternately located along a circumference of the torque rotor insertion part 239.
Here, the upper stator 231 and the upper magnetic body 236 connected with the upper stator 231 is a steel plate magnetized as an N pole and the lower stator 232 and the lower magnetic body 235 connected with the lower stator 232 is a steel plate magnetized as an S pole. Herein, a location of the N pole and S pole can be switched.
The magnetic charge detection unit 240 is fitted in one side of an outer peripheral surface of the position rotor 230 and includes a concentrator 242 provided vertically between the upper stator 231 and lower stator 232 of the position rotor 230. The magnetic charge detection unit 240 is rotatable relative to the position rotor 230.
The electric control unit receives the variation quantity of magnetic flux detected in the magnetic charge detection unit and controls a driving motor based on the received variation quantity of magnetic flux to complement the steering power of the driver.
The housing 255 is divided into an upper housing 210 and a lower housing 250 which encloses the position rotor 230 and the magnetic charge detection unit 240.
A torsion bar 205 is pressed and fitted in the input shaft 180 and output shaft 140 and then is fixed by a pin. If the driver rotates the steering wheel, the input shaft 180 connected with the steering wheel rotates and the output shaft 140 connected with the input shaft 180 through the torsion bar 205 rotates in conjunction with the input shaft 180.
At this time, if torsion is generated between the input shaft 180 and output shaft 140, a difference of a rotation angle is generated between the position rotor 230 that is pressed and fitted in the output shaft 140 to be fixed and the input shaft 180.
As described above, if the difference of the rotation angle is generated, the magnetic field formed between the upper magnetic body 236 and lower magnetic body 235 that are alternately repeated along the circumference of the position rotor 230 and the magnet 225 varies. Therefore, torque can be measured by a quantity of magnetic flux collected in the concentrator 242 of the torque sensor that passes two hole IC chips 243 and 245 installed on a substrate 241.
However, in the conventional torque rotor, a plastic injection molding process for preparing a structure for assembling the input shaft after the assembling of the yoke and the magnet is required. At this time, the plastic is flowed in the yoke and the magnet so that a roundness of the magnet is deteriorated, thereby incurring a problem of generation of magnetic field declination between the magnet and position rotor.
Further, the assembling of the yoke and the magnet is not stable so that the yoke and the magnet are idling relative to each other by shock or a difference of a coefficient of temperature expansion, thereby incurring a serious problem of generation of an error of a torque measurement value or the magnet breakage.